Electrical filters have long been used in the processing of electrical signals. In particular, such electrical filters are used to select desired electrical signal frequencies from an input signal by passing the desired signal frequencies, while blocking or attenuating other undesirable electrical signal frequencies. Filters may be classified in some general categories that include low-pass filters, high-pass filters, band-pass filters, and band-stop filters, indicative of the type of frequencies that are selectively passed by the filter. Further, filters can be classified by type, such as Butterworth, Chebyshev, Inverse Chebyshev, and Elliptic, indicative of the type of bandshape frequency response (frequency cutoff characteristics) the filter provides relative to the ideal frequency response.
The type of filter used often depends upon the intended use. In communications applications, band-pass filters are conventionally used in cellular base stations and other telecommunications equipment to filter out or block RF signals in all but one or more predefined bands. For example, such filters are typically used in a receiver front-end to filter out noise and other unwanted signals that would harm components of the receiver in the base station or telecommunications equipment. Placing a sharply defined band-pass filter directly at the receiver antenna input will often eliminate various adverse effects resulting from strong interfering signals at frequencies near the desired signal frequency. Because of the location of the filter at the receiver antenna input, the insertion loss must be very low so as to not degrade the noise figure. In most filter technologies, achieving a low insertion loss requires a corresponding compromise in filter steepness or selectivity.
In commercial telecommunications applications, it is often desirable to filter out the smallest possible pass-band using narrow-band filters to enable a fixed frequency spectrum to be divided into the largest possible number of frequency bands, thereby increasing the actual number of users capable of being fit in the fixed spectrum. With the dramatic rise in wireless communications, such filtering should provide high degrees of both selectivity (the ability to distinguish between signals separated by small frequency differences) and sensitivity (the ability to receive weak signals) in an increasingly hostile frequency spectrum. Of most particular importance is the frequency range from approximately 800-2,200 MHz. In the United States, the 800-900 MHz range is used for analog cellular communications. Personal communication services (PCS) are used in the 1,800 to 2,200 MHz range.
Microwave filters are generally built using two circuit building blocks: a plurality of resonators, which store energy very efficiently at a resonant frequency (which may be a fundamental resonant frequency f0 or any one of a variety of higher order resonant frequencies f1-fn); and couplings, which couple electromagnetic energy between the resonators to form multiple reflection zeros providing a broader spectral response. For example, a four-resonator filter may include four reflection zeros. The strength of a given coupling is determined by its reactance (i.e., inductance and/or capacitance). The relative strengths of the couplings determine the filter shape, and the topology of the couplings determines whether the filter performs a band-pass or a band-stop function. The resonant frequency f0 is largely determined by the inductance and capacitance of the respective resonator. For conventional filter designs, the frequency at which the filter is active is determined by the resonant frequencies of the resonators that make up the filter. Each resonator must have very low internal resistance to enable the response of the filter to be sharp and highly selective for the reasons discussed above. This requirement for low resistance tends to drive the size and cost of the resonators for a given technology.
For purposes of size reduction, filters often take the form of thin-filmed monolithic structures that are fabricated by depositing metal traces (making up the transmission lines of the resonators) on one side of a dielectric substrate and an insulator on the other side of the dielectric substrate. Historically, filters have been fabricated using normal; that is, non-superconducting conductors. In the case of monolithic filters, the metal traces would be composed of non-superconducting material. These conductors have inherent lossiness, and as a result, the circuits formed from them have varying degrees of loss. For resonant circuits, the loss is particularly critical. The quality factor (Q) of a device is a measure of its power dissipation or lossiness. For example, a resonator with a higher Q has less loss. Resonant circuits fabricated from normal metals in a microstrip or stripline configuration typically have Q's at best on the order of four hundred. With the discovery of high temperature superconductivity in 1986, attempts have been made to fabricate electrical devices from high temperature superconductor (HTS) materials. The microwave properties of HTS's have improved substantially since their discovery. Epitaxial superconductor thin films are now routinely formed and commercially available.
Currently, there are numerous applications where microstrip narrow-band filters that are as small as possible are desired. This is particularly true for wireless applications where HTS technology is being used in order to obtain filters of small size with very high resonator Q's. The filters required are often quite complex with perhaps twelve or more resonators along with some cross couplings. Yet the available size of usable substrates is generally limited. For example, the wafers available for HTS filters usually have a maximum size of only two or three inches. Hence, means for achieving filters as small as possible, while preserving high-quality performance are very desirable. In the case of narrow-band microstrip filters (e.g., bandwidths of the order of 2 percent, but more especially 1 percent or less), this size problem can become quite severe. In a conventional filter design, the resonators are constructed such that they operate at their fundamental resonant frequency (i.e., their lowest fundamental frequency) in order to minimize the size of the filter, as well as to prevent any undesired lower frequency re-entrant resonant frequencies that could potentially pass noise that may interfere with the desired signal.
Though microwave structures using HTS materials are very attractive from the standpoint that they may result in relatively small filter structures having extremely low losses, they have the drawback that, once the current density reaches a certain limit, the HTS material saturates and begins to lose its low-loss properties and will introduce non-linearities in the form of intermodulation distortion. For this reason, HTS filters have been largely confined to quite low-power receive only applications. However, some work has been done with regard to applying HTS to more high-power applications. This requires using special structures in which the energy is spread out, so that a sizable amount of energy can be stored, while the boundary currents in the conductors are also spread out to keep the current densities relatively small.
In one technique of filter design, the resonators are constructed such that they operate a higher order resonant frequency in order to increase the size of the structure. In this manner, the current densities in the resonators are more spread out, thereby minimizing the maximum current peaks and allowing more power to be injected into the filter while maintaining the desired levels of intermodulation distortion. Further details of such higher order filter designs are disclosed in U.S. patent application Ser. No. 12/118,533, entitled Zig-Zag Array Resonators for Relatively High Power HTS Applications” (now U.S. Pat. No. 7,894,867), and U.S. patent application Ser. No. 12/410,976, entitled “Micro-miniature Monolithic Electromagnetic Resonators” (now abandoned), which are expressly incorporated herein by reference.
For example, with reference to FIG. 1, a monolithic, bandpass, radio frequency (RF) filter 10 includes an input terminal (pad) 12, an output terminal (pad) 14, and a plurality of resonators 16 (in this case, fourteen to create fourteen poles) coupled to each other in cascade (i.e., in series) via couplings 18 between the input and output terminals 12, 14. The filter 10 further comprises a substrate 20 on which the terminals 12, 14, resonators 16, and couplings 18 are disposed. In the illustrated embodiment, each of the resonators 16 has a folded transmission line in the form of a spiral-in spiral-out (SISO) pattern, such as those described in U.S. patent application Ser. No. 12/410,976, which has previously been incorporated herein by reference. The nominal length of each transmission line is such that the respective resonator 16 has a second order resonant frequency equal to a desired pass band centered at 835 MHz, as shown in the measured frequency response plot illustrated in FIG. 2. An undesirable first order re-entrant resonant frequency is also shown in FIG. 2.
Significantly, designing the pass band of a filter around higher order resonant frequencies results in undesirable re-entrant resonances lower in frequency than the desired pass band, as well as re-entrant resonant frequencies closer to the pass band at higher frequencies than if the pass band of the filter was designed around the fundamental resonant frequency. The filter 10 has an undesirable lower order re-entrant resonant frequency at of 546 MHz, as shown in the narrowband measured frequency response plot illustrated in FIG. 3, and a desired passband centered at 835 MHz and an undesirable higher order re-entrant resonant frequencies at 1640 MHz, 1920 MHz, 2700 MHz, and 3000 MHz, as shown in the broadband measured frequency response plot illustrated in FIG. 4. The existence of re-entrant resonances in the filter 10 can lead to de-sensitization of a receiver in which the filter 10 is incorporated or unwanted interference if the signal levels at those resonances pass through the filter 10.
There, thus, remains a need to provide a filter that exhibits a considerable increase in power handling over that of typical HTS resonators, while having minimal undesired re-entrant resonant frequencies.